Luminescent ions in silica-based optical fibers
Bernard Dussardier*, Wilfried Blanc and Gérard Monnom
Laboratoire de Physique de la Matière Condensée, Université de Nice Sophia-Antipolis CNRS, UMR 6622, Parc Valrose - F-06108 NICE CEDEX 2 – France
* Corresponding author: email: [email protected], tél: +33 492 076 748
Abstract
We present some of our research activities dedicated to doped silica-based optical fibers,
aiming at understanding the spectral properties of luminescent ions, such as rare-earth and
transition metal elements. The influence of the local environment on dopants is extensively
studied: energy transfer mechanisms between rare-earth ions, control of the valence state of
Chromium ions, effect of the local phonon energy on Thulium ions emission efficiency and
broadening of Erbium ions emission induced by oxide nanoparticles. Knowledge of these
effects is essential for photonics applications.
keywords
optical fiber, silica, spectroscopy, rare-earths, transition metals, energy transfer, valence
state, phonon energy, local environment
Introduction
During the last two decades, the development of sophisticated optical systems and
devices based on fiber optics have benefited from the development of very performant optical
fiber components. In particular, optical fibers doped with 'active' elements such as rare-earth
(RE) ions have allowed the extremely fast development of optical telecommunications [i,ii],
lasers [iii] industries and the development of temperature sensors [iv]. The most frequently used
RE ions (Nd3+, Er3+, Yb3+, Tm3+) have applications in three main spectral windows: around 1,
1.5 and 2µm in fiber lasers and sensors based on absorption/fluorescence and around 1.5 µm for
telecommunications and temperature sensors. RE-doped fibers are either doped with one
element (e.g. Er3+ in line amplifiers for long haul telecommunications) or two elements (e.g.
Yb3+ and Er3+ in booster amplifiers or powerful 1.5 µm lasers). In the second case, the nonradiative energy transfer mechanism from donor to acceptor is implemented to benefit from the
good pump absorption capacity of the donor (e.g. Yb3+ around 0.98 µm) and from the good
stimulated emission efficiency of the acceptor (e.g. Er3+ around 1.5 µm). All the developed
applications of amplifying optical fibers are the result of long and careful optimization of the
material properties, particularly in terms of dopant incorporation in the glass matrix,
transparency and quantum efficiency.
The exploited RE-doped fibers are made of a choice of glasses: silica is the most widely
used, sometimes as the result of some compromises. Alternative glasses, including low
Maximum Phonon Energy (MPE) ones, are also used because they provide better quantum
efficiency or emission bandwidth to some RE ions particular optical transition. The icon
example is the Tm3+-Doped Fiber Amplifier (TDFA) for telecommunications in the S-band
(1.48-1.53 µm) [v], for which low MPE glasses have been developed: oxides [vi,vii], fluorides
[viii], chalcogenides [ix]… However, these glasses have some drawbacks not acceptable at a
commercial point of view: high fabrication costs, low reliability, difficult connection to silica
components and, in the case of fiber lasers, low optical damage threshold and resistance to heat.
To our knowledge, silica glass is the only material able to meet most of applications
requirements, and therefore the choice of vitreous silica for the active fiber material is of critical
importance. However a pure silica TDFA would suffer strong non-radiative de-excitation
(NRD) caused by multiphonon coupling from Tm3+ ions to the matrix. Successful insulation of
Tm3+-ions from matrix vibrations by appropriate ion-site 'engineering' would allow the
development of a practical silica-based TDFA.
Other dopants have recently been proposed to explore amplification over new wavelength
ranges. Bi-doped glasses with optical gain [x] and fiber lasers operating around 1100-1200 nm
have been developed [xi,xii], although the identification of the emitting center is still not clear,
and optimization of the efficiency is not yet achieved. Transition metal (TM) ions of the Ti-Cu
series would also have interesting applications as broad band amplifiers, super-fluorescent or
tunable laser sources, because they have in principle ten-fold spectrally larger and stronger
emission cross-sections than RE ions. However, important NRD strongly reduces the emission
quantum efficiency in silica. Bi- and TM-doped fibers optical properties are extremely sensitive
to the glass composition and/or structure to a very local scale. As for Tm3+ ions, practical
applications based on silica would be possible when the 'ion site engineering' will be performed
in a systematic approach. This approach is proposed via 'encapsulation' of dopants inside glassy
or crystalline nanoparticles (NP) embedded in the fiber glass, like reported for oxyfluoride
fibers [xiii] and multicomponent silicate fibers [xiv]. In NP-doped-silica fibers, silica would act
as support giving optical and mechanical properties to the fiber, whereas the dopant
spectroscopic properties would be controlled by the NP nature. The NP density, mean diameter
and diameter distribution must be optimized for transparency [xv].
In this context, our group has made contributions in various aspects introduced above.
Our motivations are both fundamental and application oriented. First, the selected dopants act as
probes of the local matrix environment, via their spectroscopic variations versus ligand field
intensity, site structure, phonon energy, statistical proximity to other dopants,… The studies are
always dedicated to problems or limitation in applications, such as for Erbium-Doped Fiber
Amplifier (EDFA) and TDFA, or high temperature sensors. It is also important to use a
commercially derived fabrication technique, here the Modified Chemical Vapor Deposition
(MCVD), to assess the potential of active fiber components for further development.
The aim of this paper is reviewing our contributions to improving the spectroscopic
properties of some RE and TM ions doped into silica. The article is organized as follows:
Section 0 describes the MCVD fabrication method of preform and fiber samples, and the
common characterization techniques used in all studies. Section 0 is devoted to the study of
energy transfers in Erbium ion (Er3+) and Ytterbium:Erbium (Yb3+:Er3+) heavily (co)doped
fibers and the applications to fiber temperature sensors, whereas Section 0 summarizes our
original investigations on Chromium (Cr3+ and Cr4+) in silica-based fibers. In Section 0, we
report on the spectroscopic investigations of Thulium- (Tm3+) doped fibers versus the material
composition, including phonon interactions and non-radiative relaxations. In section 0 are
reported our recent discoveries in RE-doped dielectric nanoparticles, grown by phase
separation.
Experimental
Preforms and fibers fabrication
All the fibers investigated in this article were drawn from preforms prepared by the
Modified Chemical Vapor Deposition (MCVD) technique [xvi] at Laboratoire de Physique de la
Matière Condensée (Nice). In this process, chemicals (such as O2, SiCl4) are mixed inside a
glass tube that is rotating on a lathe. Due to the flame of a burner moving along the tube, they
react and extremely fine silica particles are deposited on the inner side of the tube. These soot
are transformed into a glass layer (thickness is about few µm) when the burner is passing over.
The cladding layers are deposited inside the substrate tube, followed by the core layers.
Germanium and Phosphorus can be incorporated directly through the MCVD process. They are
added to raise the refractive index. Moreover, this last element is also added as a melting agent,
decreasing the melting temperature of the glass. All the other elements (rare-earths, transition
metals, aluminium, …) are incorporated through the solution doping technique [xvii]. The last
core layer is deposited at lower temperature than the preceding cladding layers, so that they are
not fully sintered and left porous. Then the substrate tube is filled with an alcoholic solution of
salts and allowed to impregnate the porous layers. After 1-2 hours, the solution is removed, the
porous layer is dried and sintered. When the deposition is complete, the tube is collapsed at
2000°C into a preform. In our case, the typical length of the preform is about 30 cm and the
diameter is 10 mm. The preform is then put into a furnace for drawing into fiber. The preform
tip is heated to about 2000°C. As the glass softens, a thin drop falls by gravity and pulls a thin
glass fiber. The diameter of the fiber is adjusted by varying the capstan speed. A UV-curable
polymer is used to coat the fiber.
Material characterizations
Refractive index profiles (RIP) of the preforms were measured using a York Technology
refractive index profiler (P101), while the RIPs of the optical fibers were determined using a
York Technology refractive index profiler (S14). The oxide core compositions of the samples
were deduced from measurement of the RIP in the preform, knowing the correspondence
between index rising and AlO3/2, GeO2, PO5/2 concentration in silica glass from the literature
[xviii,xix]. The composition was also directly measured on some preforms using electron probe
microanalysis technique in order to compare results. A good agreement was found. The
concentration of these elements is generally around few mol%. Luminescent ions concentrations
are too low to be measured through the RIP. They were measured through absorption spectra.
For example, Tm3+ ion concentration has been deduced from the 785 nm (3H6=>3H4) absorption
peak measured in fibers and using absorption cross-section reported in [xx]: abs(785 nm) =
8.7x10-25 m2.
Energy transfers in Er3+ and Yb3+:Er3+ heavily doped silica fibers
The non-radiative energy transfer processes are well-known phenomena that influence the
optical properties of doped-materials. The first theoretical basis appears in the 50's with the
Förster-Dexter's model [xxi,xxii] that treats this process as the result of dipole-dipole and
multipole-multipole interactions. Two energy transfer processes are described in Fig. 1. When
pumping a co-doped material some ions are promoted in one of their excited level. If some ions
are close to each other, their wave-functions interpenetrate and the energy stored in the excited
level of the donor ions is non-radiatively transferred to a resonant level of the acceptor ions.
This process was turned to good account in Yb3+:Er3+ co-doped silica fibers for high power fiber
amplifier [xxiii] and laser [xxiv] applications : it takes advantage of the strong absorption crosssection of Yb3+ at 980 nm and of the high efficiency of the energy transfer. In the case of high
doping levels for both species, another non-radiative energy transfer process can take place and
allows exciting a higher level of the acceptor ion : it is the double energy transfer process (DET)
described by Auzel [xxv]. This process was first used to convert infrared light from LED to
visible emission or to detect weak infrared signals with photomultipliers [xxvi,xxvii].
Double energy transfer in Er3+-doped fibers
The clustering effect in Er3+-doped silica fibers is now a well-accepted phenomenon, and
its detrimental influence on the 1550-nm gain transition of such fibers is well established
[xxviii]. For simplicity, modeling of clusters has consisted of considering that a fraction of the
dopants were organized in ion pairs [xxix], in which an immediate energy transfer leads to an
instantaneous relaxation of one excited ion. This model is in very good agreement with the
experimental results obtained for saturable absorption and for gain measurements at low Er 3+
doping levels as in fiber amplifiers. At higher doping levels, Ainslie et al. [xxx] showed that, in
addition to the ions dispersed in the host, regions in which concentrations of rare-earth
exceeding 40 wt% - called clusters – appear : in such a material the ion-pair model cannot be
applied. We have developed a cluster model [xxxi] that differs from the ion-pair one by the fact
that we consider that each ion of a cluster can efficiently transfer its energy to any of the other
ions of the same cluster. When n ions of a cluster are excited, a succession of (n-1) fast
relaxations by energy transfer leads to a situation in which all the ions of a cluster but one are
de-excited. This model permits the determination of the proportion of the dopants organized in
clusters and the transfer rate. In order to validate the model we realized a pump-absorptionversus-pump-power experiment with two fiber samples, Er-1 and Er-2, doped with 100 ppm
and 2,500 ppm of Er3+, respectively (Fig. 2). This shows that the non-saturable absorption
(NSA) grows dramatically with the Er3+ concentration. We have attributed this behaviour to the
presence of clusters containing a significant percentage of the dopants and in which efficient
energy transfers allow these ions to relax rapidly after the absorption of a first pump photon.
Double energy transfer in highly Yb3+:Er3+-co-doped fibers
The green fluorescence of Er3+-doped optical fiber is a well-known phenomenon in 800
nm-pumped erbium-doped fibers. This emission results from the excited state absorption
phenomenon and is characteristic of the emission from the 2H11/2 and 4S3/2-levels and
consequently can be observed with any pumping scheme leading to the population of these
levels. We have studied how these levels can be excited by DET [xxxii] and a schematic energy
diagram is shown in Fig. 3.
At low rare-earth concentrations, the large inter-ionic distances permit efficient single
energy transfers, but the second energy transfer is very inefficient. For applications in which the
green fluorescence is desirable, this second energy transfer must be enhanced. For that the rareearth concentration must be as high as possible to reduce the distance between neighboring ions.
In this case, a second phase, referred to as clusters, can appear in which the rare-earth ions
concentration is particularly high. In order to quantify the fraction of active ions into clusters,
we have studied the Yb3+ and Er3+ fluorescence dynamics in a highly co-doped fiber
([Yb]=[Er]=2,500 ppm) : the 1040 nm-fluorescence decay represents the population decay from
the 2F5/2-metastable level of Yb3+, and that of the green-fluorescence represents the evolution of
the 2H11/2 and 4S3/2-populations of Er3+. Our setup allows simultaneous measurements of the
counter-propagative visible emission and the lateral infrared emission. The experimental curves
show two typical decays. Fitted with our rate equations model [xxxii], they revealed that
roughly 50% of both ions are organized in clusters in the co-doped fiber. This high percentage
must be associated with very high Yb-Er transfer rates (3x106 s-1), one order of magnitude
superior to the Er3+:4I11/2 intermediate level relaxation rate (3.7x105 s-1): Er3+ ions placed in their
short lived 4I11/2 state have a higher probability to be excited to the 4F7/2 upper state than to relax
spontaneously. The strong percentage of ions organized in clusters and the very high transfer
rates are at the origin of the very good up-conversion efficiency.
Thermalization effects between excited levels in doped fibers: temperature
sensor based on fluorescence of Er3+
Though the rare-earth ions are never in thermodynamical equilibrium because of the
metastability of some levels, it has been demonstrated that the populations of the 2H11/2 and 4S3/2
levels responsible for the green emission in Er3+-doped fibers are in quasi-thermal equilibrium.
This effect has been observed for the first time in fluoride glass fibers [xxxiii] and can be
attributed to the relatively long lifetime of these levels (400 µs) in that host. In silica, in spite of
the two orders of magnitude shorter lifetime, a fast thermal coupling between both levels has
been proposed [xxxiv] and confirmed experimentally [xxxv] (Fig. 4). Indeed these levels can be
considered to be in quasi-thermal equilibrium, because of the small energy gap between them,
about 800 cm-1, compared to the high energy gap between them and the nearest lower level,
about 3000 cm-1. In this case, the lifetime of these levels is sufficient (1 µs) to allow populating
the upper level from the lower one by phonon induced transitions. Therefore R, the ratio of the
intensities coming from both levels, can be written as:
   H   H expE / kT
R
I S    S    S 
I 2H11/2
4
2
2
11/ 2
e
4
3/ 2
11/2
4
3/2
e
3/2
(1)
where  is the frequency, e the emission cross section, k the Boltzmann constant, E

the energy gap between the two levels and T the temperature in degrees Kelvin. In Fig. 4 we
show that the experimental data can be fitted by a function in agreement with Equation (1). This
is another example of an energy transfer process, this one being assisted by phonons.
In order to take advantage of the high efficiency of the DET in highly co-doped Yb-Er
doped fiber and of the thermalisation effect between the higher levels involved in the green
fluorescence in this kind of fiber, we have developed a new temperature sensor, unsensitive for
strain. The dynamic obtained was 11 dB in Fig. 4 over the shown temperature range, leading to
a mean rate of change of the green intensity ratio of approximately 0.016 dB/K at 300 K.
Several temperature cycles have been carried out and we have observed a good repeatability. As
for the stability, no modifications have been observed on the two intensities when the fiber was
heated during several hours at temperatures up to 600°C. Due to the strong absorption of the
doped fiber in the signal wavelength range - the green emissions corresponding to transitions
downto the fundamental level - and to the 15 dB/km intrinsic absorption of the transparent fiber
in the same wavelength range, such a device would be limited to a point sensor.
We have developed a new sensor based on the 1.13 µm and 1.24 µm emission lines,
coming from the same levels [xxxvi]. These lines present the same temperature behaviour as the
green ones. As the lower level of these transitions is the 4I11/2-level and not the fundamental one
(Fig. 3), the signals are absorption free and their wavelengths correspond to a transparency
region of the intermediate fibers. These arguments have permitted the development of an
efficient quasi-distributed configuration without limitation on the sensing line length : the short
lifetime of upper levels (1 µs) could allow realizing a sensors network. Each sensitive head is
separated from its neighbors by a 100-meter long transparent silica fiber in order to time-resolve
the counter-propagative signals.
Conclusion
Energy transfer processes in rare-earth-doped materials have been studied since the
middle of the 20th century. At the beginning, the applications of DET were mainly conversion of
infrared light to visible emission or detection of weak infrared signals with photomultipliers. A
renewal interest appears with the development of optical fibers in which high power density can
be achieved: single energy transfer allows improvement of high power fiber amplifiers and laser
and DET permits realizing point and quasi-distributed fiber sensors.
Local structure, valency states and spectroscopy of transition metal ions
Optical fiber materials with very broad-band gain are of great interest for many
applications. Tunability in RE-doped fiber devices is already well established, but limited by
shielding of the optically active electronic orbitals of RE ions. Optically active, unshielded
orbitals are found in transition metal (TM) ions. Some TM-doped bulk solid-state lasers
materials, such as Cr4+:YAG, have demonstrated very good results as broad-band gain media
[xxxvii]. Tentatives with other TM ions, like Ni2+ in vitroceramics fiber are also promising
[xiv]. More recently, a 400-nm emission bandwidth was observed from a fiber whose Cr-doped
core was made of Y2O3:A2O3:SiO2 obtained by a rod-in-tube technique using a Cr4+:YAG rod as
core material and a silica tube as cladding material [xxxviii].
Little literature exists on chromium- and other TM-doped vitreous bulk silica, although
this issue was addressed in the 70’s [xxxix] to improve transmission of silica optical fibers.
Some reports on chromium-doped glasses have already shown evidence of absorption and nearinfrared (NIR) fluorescence due to Cr4+ in these materials [xl,xli]. However their compositions
and preparation techniques greatly differ from those of silica optical fibers. Therefore, some
basic studies on the optical properties of TM ions in silica-based optical fibers are needed. In
particular, the final TM oxidation state(s) in the fiber core strongly depend(s) on the preparation
process. Also, the optical properties (absorption and luminescence) of one particular oxidation
state of a TM ion varies from one host composition and structure to another, due to variations of
the crystal-field (so-called ligand field in glass) [xlii]. Hence the interpretation of absorption and
emission spectroscopy is difficult. Because no luminescence spectroscopy of the TM-doped
silica fibers had been reported before, we have contributed to explore this field. We have
studied the influence of the chemical composition of the doped region on the Cr-oxidation states
and on the spectroscopic properties of the samples. We have also studied the optical properties
versus the experimental conditions (temperature and pump wavelength). We describe the
experimental details specifically used for TM-doped fibers, then we summarize all results and
interpretations.
Fabrication and characterization of Chromium-doped samples
The preforms and fibers were prepared as described in §0 Chapitre:, using Cr3+-salt
alcoholic doping solution and oxygen or nitrogen (neutral) atmosphere for the drying-tocollapse stages. Three different types of samples containing Ge or/and Al were prepared,
referred to as Cr(Ge), Cr(Ge-Al) and Cr(Al), respectively. The total chromium concentration
([Cr]) was varied from below 50 mol-ppm to several thousands mol ppm. Above several 100s
mol ppm, preform samples had evidence of phase separation causing high background optical
losses, whereas fibers (few 10s mol ppm) did not show phase separation and had low
background losses (<1 dB/m). The oxidation states of Cr and their relative concentrations were
analyzed by Electron Paramagnetic Resonance, whereas the absolute content of all elements
(including Cr) was analyzed by Plasma Emission Spectroscopy.
Absorption spectra were analyzed using the Tanabe-Sugano (T.-S.) formalism [xliii] to
compare our assignments to optical transitions with reports on Cr3+- and Cr4+-doped materials.
This formalism helps predicting the energy of electronic states of a TM in a known ligand field
symmetry as a function of the field strength Dq and the phenomenologic B and C so-called
Racah parameters (all in cm-1, Fig. 5). The Dq/B ratio allows the qualitative determination of
some optical properties of TM ions, such as strength, energy and bandwidth of optical
transitions. We have also estimated the absorption cross-sections using results from composition
and valency measurements. Absorption and emission spectroscopies including decay
measurements were performed on both preforms and fibers, at room temperature (RT) and low
temperature (LT, either 12 or 77 K), using various pump wavelengths: 673, 900 and 1064 nm.
Full details of the experimental procedures are given in [xliv,xlv,xlvi].
Principal results
By slightly modifying the concentration in germanium and/or aluminium in the core of
the samples, their optical properties are greatly modified. In particular, we have shown that:
i)
Only Cr3+ and Cr4+ oxidation states are stabilized. Cr3+ is favoured by Ge co-doping, and
lies in octahedral site symmetry (O), as in other oxide glasses [xlvii]. Cr4+ is present in all
samples. This valency is promoted by Al co-doping or when [Cr] is high, and lies in a distorded
tetrahedral site symmetry (Cs) [xlviii,xlix]. The low-doped Cr(Al) samples contain only Cr4+ and
their absorption spectra are similar to those of aluminate [xl] and alumino-silicate glasses [xli]
and even crystalline YAG (Y3Al5O12) [l]. Glass modifiers like Al induce major spectroscopic
changes, even at low concentrations (~1-2 mol%). This would help engineering the Chromium
optical properties in silica-based fibers, using possibly alternative modifiers.
ii)
The absorption spectra have been interpreted and optical transitions assigned for each
present valency state (Fig. 6). The absorption cross sections curves (abs) were estimated. For
Cr3+, abs(Cr3+, 670 nm)= 43 x 10-24 m2 is consistent with reported values in other materials,
such as ruby [li] and silica glass [xxxix], while abs(Cr4+, 1000 nm)~3.5 x 10-24 m2 is lower than
in reference crystals for lasers [lii] or saturable absorbers [liii], but consistent with estimated
values in alumino-silicate glass [xli].
iii) Using the T.-S. formalism, we found Dq/B = 1.43 which is lower than the value were
3
T2 and 1E levels cross (Dq/B = 1.6, Fig. 5). As a consequence, the expected emission is along
the 3T23A2 transition as a broad featureless NIR band. No narrow emission line from the 1E
state is expected, in agreement with fluorescence measurements. Dq/B is lower than those
reported for Cr4+ in laser materials like YAG and Forsterite [xlviii].
iv) The LT fluorescence from Cr4+ spreads over a broad spectral domain, from 850 to
1700 nm, and strongly varies depending on core chemical composition, [Cr] and p (pump
wavelength). The observed bands were all attributed to Cr4+ ions, in various sites. Fig. 7 shows
the fluorescence spectra of Cr4+ in two different types of samples and in various experimental
conditions. Possible emission from other centers (Cr3+, Cr5+, Cr6+) was discussed, but rejected
[xlvi]. The fluorescence sensitivity to [Cr] and p suggests that Cr-ions are located in various
host sites, and that several sites are simultaneously selected by an adequate choice of p (like in
Cr(Ge-Al)). It is also suggested that although Al promotes Cr4+ over Cr3+ when [Cr] is low, Cr4+
is also promoted in Ge-modified fibers at high [Cr].
v)
The strong decrease of fluorescence from LT to RT is attributed to temperature
quenching caused by multiphonon relaxations, like in crystalline materials where the emission
drops by typically an order of magnitude from 77 K to 293 K [xxxvii].
vi) The LT fluorescence decays are non-exponential (Fig. 8) and depend on [Cr] and s.
The fast decay part is assigned to Cr clusters or Cr4+-rich phases within the glass. The 1/elifetimes ( at s = 1100 nm are all within the15-35 s range in Al-containing samples,
whereas  ~ 3-11 s in Cr(Ge) samples, depending on [Cr]. The lifetime of isolated ions (iso),
measured on the exponential tail decay curves (not shown) reach high values: iso~200 to 300 µs
at s~1100 nm, iso~70 µs at s~1400 nm. In the heavily-doped Cr(Ge) samples, iso is an order
of magnitude less. Hence, Cr4+ ions are hosted in various sites: the lowest energy ones suffer
more non-radiative relaxations than the higher energy ones. Also presence of Al improves the
lifetime, even at high [Cr]. It is estimated that at RT, lifetime  would be of the order of 1 µs or
less. This fast relaxation time, compared to RE ions (~1 ms) has been implemented as a
fiberized saturable absorber in a passively Q-switched all-fiber laser [liv].
Conclusion
The observed LT fluorescence of Cr4+ is extremely sensitive to glass composition, total
Cr concentration and excitation wavelength. Using Aluminum as a glass network modifier has
advantages: longer excited state lifetime and broader fluorescence bandwidth than in
Germanium-modified silica. A combination of Al and Ge glass modification induces the
broadest fluorescence emission in the NIR range, to our knowledge, exhibiting a 550 nmbandwidth. However, increasing the quantum efficiency is now necessary for practical fiber
amplifiers and light sources. Further investigations concluded to the necessity of local
surrounding TM ions with a different material, i.e. having sensibly different chemical and
physical properties compared to pure silica, in order to improve the local site symmetry and
hence minimize NRD. Preliminary implementation of this principle was reported recently,
concerning Cr3+ ions in post-heat-treated Ga-modified silica fibers [lv]. When engineering of the
local dopant environment will be possible, then practical TM-doped silica-based amplifying
devices will be at hand.
Phonon interactions / non-radiative relaxations: improvement of Tm3+ efficiency
Thulium-doped fibers have been widely studied in the past few years. Because of Tm3+
ion rich energy diagram, lasing action and amplification at multiple infrared and visible
wavelengths are allowed. Thanks to the possible stimulated emission peaking at 1.47 µm (3H4
=> 3F4, see Fig. 9), discovered by Antipenko et al. [lvi], one of the most exciting possibilities of
Tm3+ ion is amplifying optical signal in the S-band (1.47–1.52 µm), in order to increase the
available bandwidth for future optical communications. Unfortunately, the upper 3H4 level of
this transition is very close to the next lower 3H5 level so non-radiative de-excitations (NRD) are
likely to happen in high phonon energy glass host, causing detrimental gain quenching.
Oxide modifiers influence on the 3H4-level lifetime
To address this problem, we have studied the effect of some modifications of Tm3+ ion
local environment. Keeping the overall fiber composition as close as possible to that of a
standard silica fiber, we expect to control the rare-earth spectroscopic properties by co-doping
with selected modifying oxides. We have studied the incorporation of modifying elements
compatible with MCVD. GeO2 and AlO3/2 are standard refractive index raisers in silica. AlO3/2 is
also known to improve some spectroscopic properties of Er3+ ion for C-band amplification [i]
and to reduce quenching effect through clustering in highly rare-earth-doped silica [lvii]. Both
oxides have a lower maximum phonon energy than silica. We use high phonon energy PO5/2 as
opposite demonstration. GeO2 and PO5/2 concentrations are 20 and 8 mol%, respectively. AlO3/2
concentration is varied from 5.6 to 17.4 mol%. Tm3+ concentration is less than 200 mol ppm.
To investigate the role of the modification of the local environment, decay curves of the
810 nm fluorescence from the 3H4 level were recorded. All decay curves measured are nonexponential. This can be attributed to several phenomena and will be discussed in this article.
Here, we study the variations of 1/e lifetimes () versus concentration of oxides of network
modifiers (Al or P) and formers (Ge). The lifetime strongly changes with the composition of the
glass host. The most striking results are observed within the Tm(Al) sample series:  linearly
increases with increasing AlO3/2 content, from 14 µs in pure silica to 50 µs in sample Tm(Al)
containing 17.4 mol% of AlO3/2. The lifetime was increased about 3.6 times. The lifetime of the
20 mol% GeO2 doped fiber Tm(Ge) was increased up to 28 µs whereas that of the 8 mol% PO5/2
doped fiber Tm(P) was reduced down to 9 µs. We see that aluminum codoping seems the most
interesting route among the three tested codopants.
Non-exponential shape of the 810-nm emission decay curves
All fluorescence decay curves from the 3H4 level are non-exponential. We have
investigated the reasons for this non-exponential shape in various silica glass compositions. We
observed that the decay curve shape depends only on the Al-concentration, even in the presence
of Ge or P in samples Tm(Ge) and Tm(P), respectively [lviii]. It is thought that Tm3+ ions are
inserted in a glass which is characterized by a multitude of different sites available for the rareearth ion, leading to a multitude of decay constants. This phenomenological model was first
proposed by Grinberg et al. and applied to Cr3+ in glasses [lix]. Here we apply this model, for
the first time to our knowledge, to Tm3+-doped glass fibers. In this method, a continuous
distribution of lifetime rather than a number of discrete contributions is used. The advantage of
this method is that no luminescence decay model or physical model of the material is required a
priori. The luminescence decay is given by:

I(t)   Ai exp t /  i
i

(2)
where A() is the continuous distribution of decay constant.

The procedure for calculating  and the fitting algorithm are described in detail in
[lix]. For the fitting procedure, we considered 125 different values for i, logarithmically spaced
from 1 to 1000 µs. By applying this procedure to all the decay curves, a good matching was
generally obtained. For a given composition (Fig. 10), we can notice two main distributions of
the decay constant. With the aluminium concentration, they increase from 6 to 15 µs and from
20 to 50 µs, respectively. For the highest aluminium concentration (9 mol%, in Tm(Ge) and
Tm(P)), these two bands are still present (not shown in the figure). One is around 10 µs and the
second one spreads from 30 to 100 µs, for both compositions (Tm(Ge) and Tm(P)). According
to the phenomenological model, the width of the decay constants distribution is related to the
number of different sites. The large distribution around 80 µs is then due to a large number of
sites available with different environments. It is however remarkable that this distribution at 80
µs is very similar in both sample types. From the Tm3+ ion point of view (considering
luminescence kinetics), Tm(Ge) and Tm(P) glasses seem to offer the same sites.
The meaning of the decay constant values is now discussed. Lifetime constants obtained
from the fitting can be correlated with the one expected for Thulium located in a pure silica or
pure Al2O3 environment. The 3H4 lifetime is calculating by using this equation:
1   1  rad Wnr
(3)
where rad corresponds to the radiative lifetime which is given to be 670 µs in silica [lx].

Wnr is the non-radiative decay rate, expressed as [lxi]:


Wnr  W0  exp  (E  2Ep )
(4)
where W0 and  are constants depending on the material, E is the energy difference
 3H5 levels and Ep is the phonon energy of the glass. W0 and  were
between the 3H4 and
estimated for different oxide glasses [lxi, lxii]. The energy difference E was estimated by
measuring the absorption spectrum of the fibers. When Al concentration varies, this value is
almost constant around 3700 cm-1 [lxiii].
With these considerations, the 3H4 expected lifetime can be calculated. In the case of
silica glass, silica = 6 µs and for an Al2O3 environment, alumina = 110 µs. These two values are in
accordance with the ones we obtained from the fitting procedure. The distribution of decay
constant around 10 µs corresponds to Tm3+ ions located in almost pure silica environment while
the second distribution is attributed to Tm3+ located in Al2O3 -rich sites.
Conclusion
By adding oxide network modifiers or formers, we demonstrated that aluminium is the
most efficient to improve the 3H4 level-lifetime. This was attributed to a lower local phonon
energy. Potential of the amplification in the S-band was then investigated. In the fiber with the
highest aluminium concentration, gain curve was measured. Although excitation wavelength
(1060 nm), refractive index profile and Thulium concentration were not optimized, a gain of 0.9
dB was obtained at 1500 nm [lxiv]. With a numerical model of the TDFA that we developed
[lxv], we estimated that a gain higher than 20 dB is reachable in a silica-based TDFA.
Rare-earth-doped dielectric nanoparticles
Erbium-doped materials are of great interest in optical telecommunications due to the Er3+
intra-4f emission at 1.54 µm. Erbium-Doped Fiber Amplifiers (EDFA) were developed in silica
glass because of the low losses at this wavelength and the reliability of this glass. Developments
of new rare-earth doped fiber amplifiers aim to control their spectroscopic properties: shape and
width of the gain curve, optical quantum efficiency, .... Standard silica glass modifiers, such as
aluminum, give very good properties to available EDFA. However, for more drastic
spectroscopic changes, more important modifications of the rare-earth ions local environment
are required. To this aim, we present a fiber fabrication route creating rare-earth doped calcosilicate or calco-phospho-silicate nanoparticles (NP) embedded in silica glass.
Nanostructured fibers preparation
In the chosen route, NP are not prepared ex-situ and incorporated into the perform. To
prepare them, we take advantage of the heat treatement occurring during the MCVD process.
Their formation is based on the basic principle of phase separation. On the basis of
thermodynamical data such as activity coefficient, entropy of mixing, enthalpy of mixing and
Gibbs-free energy change, the phase diagram of the SiO2-CaO binary compound was derived
using Factstage software (Fig. 11). A miscibility gap is found when the CaO concentration is
between 2 and 30 mol%. In this region, CaO droplets are formed, like oil in water. Such
phenomenon is expected during perform fabrication as temperature reaches 2000°C during
collapsing passes.
For Calcium doping, CaCl2 salt was added to the Er3+ containing soaking solution. Four
CaCl2 concentrations were studied (0, 0.001, 0.1 and 1 mol/l). Ge and P were also added by
MCVD. When the Ca concentration was increased in the doping solution, the aspect of the
central core of the preform turned from transparent to milky. This variation is explained by the
structural changes of the core. For preforms with calcium concentration higher than 0.01 mol/l,
NP were observed by Transmission Electron Microscopy (TEM) on preform samples (Fig. 12).
We can clearly observe polydisperse spherical NP with an estimated mean diameter of 50 nm.
Smaller particles of 10 nm are visible. The size of the biggest particles was around 200 nm (not
shown in Fig. 12). When the Ca concentration decreases, the size distribution of the particles is
nearly identical but the density is lower. The composition of the core was investigated by
Energy Dispersive X-ray analysis: the NP contained equal amounts of Ca, P and Si cations,
whereas only Si cations was detected in the surrounding matrix. Ge seemed to be
homogeneously distributed over the entire glass. The most important finding is that Er3+ ions
and Ca were detected only within the NP.
Erbium emission characterizations
Spectroscopic characterizations on the emission line associated to the 4I13/2-4I15/2 transition
at 1.54 µm were made at room temperature on Er-doped samples with (sample A) and without
(sample B) calcium. The results are shown in Fig. 13 where we evidence the fact that the
emission spectrum of sample A is broader than that of sample B. To explain these differences
we have studied the Er3+ local environment. EXAFS measurements at the Er-LIII edge (E=8358
eV) were carried out at the GILDA-CRG beamline at the European Synchrotron Radiation
Facility. In sample A the rare-earth is linked to O atoms in the first coordination shell and to Si
or P atoms (these two atoms can not be distinguished due to the similar backscattering
amplitude and phase) in the second shell in a way similar to that already observed in silicate
glasses [lxvi], phosphate glasses [lxvii]. Also the structural parameters (about 7 O atoms at 2.26
Å and Si (or P) atoms at 3.6 Å corresponding to a Er-O-Si (or P) bond angle of ≈140 deg) are in
good agreement with the cited literature. Si(or P) atoms are visible as they belong to the same
SiO4 (or PO4) tetrahedron as the first shell O atoms but no further coordination shells are
detected. This permits to state that an amorphous environment is realized around Er 3+ ions. On
the other hand sample B presents a completely different EXAFS signal that is well comparable
with the spectrum of ErPO4. This means that Er in this case is inserted in a locally well ordered
phase of about a few coordination shells (around 4-5 Å around the absorber). The fact that TEM
on this sample reveals a uniform sample is not in contradiction with this result; it just means that
this phase is not spatially extended to form nm-sized NP (in our TEM analyses the spatial
resolution is limited to few nm) but the ordering is extremely local, i.e. it is limited to only a
few shells around the rare-earth ion.
From these considerations, the broadening of the emission spectrum observed in sample A
can be attributed to an inhomogeneous broadening due to Er3+ ions located in a more disordered
environment compare to sample B. Here we see that the cumulated effects of Ca and P within
the Er-doped NP both amorphize the material structure around Er3+ ions and increase the
fluorescence inhomogeneous broadening.
Conclusion
In this paragraph, we have demonstrated that through the phase separation mechanism,
nanoparticles can be obtained in preforms by adding Calcium. Er3+ ions are found to be located
only into these nanoparticles. An inhomogeneous broadening of the emission band is observed,
associated to Er3+ ions located in a more disordered environment compare to silica. This feature
is particularly interesting in the production of materials for Wavelength Division Multiplexing
applications, such as Erbium-Doped Fiber Amplifier with a broader band gain.
Perspectives and conclusion
The choice of a glass to develop new optical fiber component is most of the time a result
of compromises. Silica glass is the most widely used for its many advantages (reliability, low
cost fabrication, …). However, it suffers from different drawbacks, such as high phonon energy
or low luminescent ions solubility, which affect quantum efficiency or emission bandwidth of
luminescent ions, for example. We have shown in several cases that spectroscopic properties of
dopants are not directly related to the average properties of the doped glass, but to their local
environment. Indeed, by slightly modifying the silica composition, we succeeded to control the
Chromium valence state and improve the Thulium emission efficiency. Moreover, we present
interest of high doping level to take advantage of energy transfers. Then, nanostructuration of
doped fiber is proposed as a new route to ‘engineer’ the local dopant environment. All these
results will benefit to optical fiber components such as lasers, amplifiers and sensors, which can
now be realized with silica glass.
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Figures captions
Fig. 1: Schematic energy diagram of (a) single energy transfer between two ions, (b) double
energy transfer.
Fig. 2 : Absorption of the Er-1 and Er-2 fibers vs launched pump power. Squares: experimental
data; solid lines: cluster model for 0%, 10% and 52% of Er3+-ions in clusters; vertical
arrows: non-saturated absorption as a difference with simulation for 0% cluster.
Fig. 3 : Energy scheme for the DET process. Level energies are in cm-1; their lifetimes between
round brackets.
Fig. 4 : Natural logarithm of measured intensity ratio (R) plotted against the inverse of
temperature.
Fig. 5: Normalized Tanabe-Sugano energy level diagram for Cr4+ in tetrahedral ligand field (Td
symmetry) showing the energy states of interest, for C/B = 4.1. The free ion states are
shown on the left of the ordinate axis. The dashed line (Dq/B = 1.43) reveals the relative
positions of the states found for Cr4+ in the silica-based samples: the first excited state
level is 3T2(3F).
Fig. 6:. Background corrected absorption from (left) a Cr(Ge) preform ([Cr] = 1400 ppm) and
(right) a Cr(Al) fiber ([Cr] = 40 ppm). Circles: experimental data; Solid lines: adjusted
bands to Cr3+ (left) and Cr4+ (right) transitions, respectively; and resulting absorption
spectra. Assignments are indicated from the ground level Cr3+:4A2 or Cr4+:3A2 to the
indicated excited level, respectively. The Cr4+:3T2 level three-fold splitting is due to
distorsion from perfect tetrahedral symmetry. The spin-forbidden Cr3+:4A22E and
Cr4+:3A21E transitions are not visible and overlapping with the intense spin-allowed
transitions.
Fig. 7: Fluorescence spectra: (a) fiber Cr(Al):[Cr] = 40 ppm, p = 900 nm, T=77 K, (b) preform
Cr(Ge-Al): [Cr] = 300 ppm, p = 673 nm, T=12K.
Fig. 8: Fluorescence decays from Cr(Al) samples, p = 673 nm, T=12 K: (a) s~1100 nm and
[Cr]=40 ppm, (b)s~1100 nm, [Cr]=4000 ppm, and (c) s~1400 nm, [Cr]=4000 ppm
Fig. 9: Schematic energy diagram of Tm3+ ion, showing the relevant multiplets. Solid arrows:
absorption and emission optical transitions; thick arrow: NRD (non-radiative deexcitation) across the energy gap between the 3H4 and 3H5 multiplets, E~ 3700 cm-1.
Fig. 10 : Histograms of the recovered luminescence decay time distributions obtained for silicabased Tm3+-doped fibers with phosphorus incorporated in the core and different Al 2O3
concentration.
Fig. 11: Miscibility-gap in the derived phase-diagram of binary SiO2-CaO glass
Fig. 12: TEM image from preform sample doped with Ca and P.
Fig. 13 : Room temperature emission spectra of Er-doped preform with (sample A) and without
(sample B) Calcium. Samples were excited at 980 nm.
Fig. 1: Schematic energy diagram of (a) single energy transfer between two ions, (b) double
energy transfer.
4
Er 2 fiber
3
52%
A (dB)
2
10%
Er 1 fiber
NSA
1
0%
Pin (mW)
0
0
10
20
30
40
50
Fig. 2 : Absorption of the Er-1 and Er-2 fibers vs launched pump power. Squares: experimental
data; solid lines: cluster model for 0%, 10% and 52% of Er3+-ions in clusters; vertical
arrows: non-saturated absorption as a difference with simulation for 0% cluster.
Fig. 3 : Energy scheme for the DET process. Level energies are in cm-1; their lifetimes between
round brackets.
Ln ( R )
1.5
0.5
-0.5
1
2 1000/ T (K) 3
Fig. 4 : Natural logarithm of measured intensity ratio (R) plotted against the inverse of
temperature.
3T (3P)
1
3T (3F)
1
E/B
60
3T (3F)
2
30
3P
1E (1D)
1D
3F
0
0
1
1.43
2
3
4
3A (3F)
2
Dq/B
Fig. 5: Normalized Tanabe-Sugano energy level diagram for Cr4+ in tetrahedral ligand field (Td
symmetry) showing the energy states of interest, for C/B = 4.1. The free ion states are
shown on the left of the ordinate axis. The dashed line (Dq/B = 1.43) reveals the relative
positions of the states found for Cr4+ in the silica-based samples: the first excited state
level is 3T2(3F).
Fig. 6:. Background corrected absorption from (left) a Cr(Ge) preform ([Cr] = 1400 ppm) and
(right) a Cr(Al) fiber ([Cr] = 40 ppm). Circles: experimental data; Solid lines: adjusted
bands to Cr3+ (left) and Cr4+ (right) transitions, respectively; and resulting absorption
spectra. Assignments are indicated from the ground level Cr3+:4A2 or Cr4+:3A2 to the
indicated excited level, respectively. The Cr4+:3T2 level three-fold splitting is due to
distorsion from perfect tetrahedral symmetry. The spin-forbidden Cr3+:4A22E and
Cr4+:3A21E transitions are not visible and overlapping with the intense spin-allowed
transitions.
Fig. 7: Fluorescence spectra: (a) fiber Cr(Al):[Cr] = 40 ppm, p = 900 nm, T=77 K, (b) preform
Cr(Ge-Al): [Cr] = 300 ppm, p = 673 nm, T=12K.
1
Intensity (a.u.)
(a)
0,1
(b)
(c)
0,01
0,001
0
100
200
300
400
Time (µs)
Fig. 8: Fluorescence decays from Cr(Al) samples, p = 673 nm, T=12 K: (a) s~1100 nm and
[Cr]=40 ppm, (b)s~1100 nm, [Cr]=4000 ppm, and (c) s~1400 nm, [Cr]=4000 ppm
3
NRD
E
H4
3
1470 nm (S-band)
H5
3
F4
785 nm
810 nm
3
H6
Fig. 9: Schematic energy diagram of Tm3+ ion, showing the relevant multiplets. Solid arrows:
absorption and emission optical transitions; thick arrow: NRD (non-radiative deexcitation) across the energy gap between the 3H4 and 3H5 multiplets, E~ 3700 cm-1.
Fig. 10 : Histograms of the recovered luminescence decay time distributions obtained for silicabased Tm3+-doped fibers with phosphorus incorporated in the core and different Al 2O3
concentration.
Fig. 11: Miscibility-gap in the derived phase-diagram of binary SiO2-CaO glass
Fig. 12: TEM image from preform sample doped with Ca and P.
Fig. 13 : Room temperature emission spectra of Er-doped preform with (sample A) and without
(sample B) Calcium. Samples were excited at 980 nm.